* The file properties of the FLAC file are imported from its Embedded Cue Sheet
and
* You issue an "Update Tags from Library" command or that is done automatically by your settings.
I believe this started with build 154, but I have not tried a variety of builds. Full description:
Bug - "Update Tags from Library" wipes all physical tag information from Single FLAC File With Embedded Cue Sheet. It does this in a destructive way that then prevents the same FLAC file from being imported correctly with an external CUE file. (So the FLAC file has been damaged in some way, at least for metadata purposes.)
Background:
If you edit File Properties of many files at once (such as changing Genre), and then do "Update Tags from Library", if this includes albums as a Single FLAC or APE file with a separate CUE file, then you will get a count of "N Failures" where N is the number of tracks in those albums. MC18 does not rewrite the CUE file that was the source of data, which is known and expected behavior. It also does not do anything to the Single APE or FLAC file (and I tested, and that is still true with 155 - I tested with a CUE and FLAC file pair and a CUE and APE file pair). So, you get the "N Failures" error message, and no physical files are changed at all.
But, if the metadata comes from an Embedded CUE sheet in a FLAC file, then "Update Tags from Library" will wipe all metadata from the FLAC file and damage it.
Note:
This is probably introduced in 154, although it could be earlier.
PS As long as we are on the subject of "Update Tags from Library", note that running it with SACD ISO files results in "0 Failures" despite the fact that the file cannot be written (and a later import from just the SACD ISO file will not include the metadata changes that "Update Tags from Library" is supposed to write. So, there is a missing error report in that case.